Borehole measurement devices may be used to determine formation characteristics surrounding the borehole and are typically used in wellbores drilled for the purpose of extracting natural resources, such as hydrocarbon, from the formations surrounding the borehole. Borehole measurement devices or logging tools may use different types of measurements, for example, a borehole measurement device may use gamma measurements, thermal neutron measurements, resistivity measurements or other types of measurements.
At present there are a number of physics being employed to perform thru pipe formation evaluation and other wellbore measurements. For neutron based measurements, there are pulsed neutron thermal neutron, pulsed neutron gamma, gamma gamma, neutron thermal neutron, neutron epi-thermal neutron etc. It is noted that naming convention for down hole geophysical devices are based on source—detection physics. For example neutron thermal neutron indicates a neutron source and thermal neutron detection. Most of these systems utilize one source per physics, which will be referred to as single physics measurement. Systems that utilize one source for two physics will be referred to as a dual physics measurement. Of the currently available dual physics systems, there is one that utilizes a combination neutron thermal neutron, neutron epi-thermal neutron which can be considered the closest analogy to the Quad Neutron dual physics measurement. In addition there is a neutron thermal neutron, neutron gamma dual physics measurement device built in Azerbaijan.
The Some shortcomings or disadvantages of using single physics measurements is the lack of corrections available for factors such as borehole rugosity, annular fluid changes, mineralogy, tubulars etc. To compensate for these short comings, dual detectors are used. These devices are commonly referred to as compensated devices. Another solution is to combine multiple single physics measurement devices, including compensated devices, during analysis. An example is neutron thermal neutron and gamma gamma physics, commonly referred to as neutron density measurements. Measuring thru pipe also limits the effectiveness of some of the physics. For example, gamma gamma measurements are limited because the pipe itself shields gamma and therefore there are losses as the gamma photons travel from the source thru the pipe and then again as the photon returns back to the detector. This results in low count rates and increases error in the measurements.
Neutrons can easily penetrate pipe and therefore is a logical choice for thru pipe measurements. Of the neutron physics based measurements; pulsed neutron devices do not lend themselves to dual physics measurements. The reason is that the length of the pulsed neutron source does not allow for effective measurement spacing for the required detectors. Chemical neutron sources are much smaller and therefore can be used effectively for dual physics based measurements. The neutron epi-thermal measurement is highly sensitive to borehole rugosity and therefore not an ideal choice to determine formation parameters. The measurement industry typically uses a single neutron source measure device to collect this data. A single neutron source device is used to achieve high neutron output. A single high neutron source is used with a generally assumed spheroid geometry for the source field with the center being the source of the neutrons. Device detectors are placed near the source as to be within the field. Typically, devices are designed to have one detector as near to the source as practically possible in an axial configuration. Generally, limitations as to proximity are mechanical in nature. However, the use of a single high output neutron source has limited accuracy with respect to the data collected relating to porosity and clay volume measurements.
Early commercial neutron logging tools comprised a single neutron source and a single neutron detector. The information derived by these tools was of rather limited value, since it did not separate the effects of formation porosity and formation salinity.
Several approaches were taken to eliminate or at least to reduce the borehole effect on the porosity reading of a neutron well logging tool. One simple way to achieve this is to design a decentralized tool with the source and detectors pressed against one wall of the borehole.
A dual neutron emission measure device has also been developed which alternates the source outputs. However, this results in a spheroid field geometry. In addition, the alternating source does not enhance the source field geometry and does not allow for an effective balanced 4 detector measurement system.
A need therefore exists for a neutron source geophysical measurement device that overcomes one or more of the shortcomings observed in the industry.